1. Field of the Invention
The present invention relates to an apparatus of optical recording, reproducing and erasing on a recording disc, and a method of optical recording, reproducing and erasing therefor.
2. Prior Art
Extensive research works have been hitherto made for realizing optical recording and reproducing disc apparatuses which comprise photosensitive recording films on optical recording disc substrates and which are capable of recording information data in the recording films and reproducing the data therefrom.
Laser light sources with high coherency have been employed as light sources used for the conventional optical recording disc apparatuses where light beams must be condensed with high reliability. It is highly expected that such laser light sources are replaced by semiconductor laser devices, which are more advantageous in realizing a more compact apparatus for optical recording discs and in having a possibility of direct light modulation.
On the other hand, as recording film materials various materials have been also studied and developed, for example thin films capable of causing localized evaporation thereof by light irradiation heating, or another thin films capable of causing localized changes in reflectivity and transmittance thereof upon light irradiation. As one of the thin films of the latter cases it has been proposed in the U.S. application filed on Aug. 26, 1974 (now patented under U.S. Pat. No. 3,971,874 for Ohta et al.) that thin films having a composition of TeO.sub.x (0&lt;x&lt;2.0) are applicable for optical recording system, by which high quality recording signals are obtainable. In the system a high recording sensitivity is obtainable, since the recording is made in a retrievable way, that is without any evaporation of component layer of the recording disc, and the thin films are stable and less sensitive in temperature and humidity, and besides obtained recording signals are of high quality.
When recording information data in the thin films, they are caused to locally change their physical nature from amorphous to crystal-like states by light irradiation. The light irradiation causes to increase the reflectivity of the thin films, which is evaporate-deposited on recording disc substrates. Optimum recording light source power is 6-9 mW for a recording disc substrate rotating at a rotational speed of 1,800 r.p.m. This means that a semiconductor laser device capable of emitting light energy of at least 20 mW is required when taken into account of a transfer efficiency for light passing through an optical system employed in the conventional optical recording disc apparatus. The most advanced semiconductor technology makes semiconductor laser devices available which are capable of stable and continuous lasing with lasing power more than 20 mW and wavelengths of about 800 nm falling into the near infrared wavelength range. Since focussed light beam radii are approximately proportional to the wavelengths of laser light, it is highly preferable to employ semiconductor laser devices capable of emitting visible light with wavelengths shorter than those in the near infrared wavelength range for the purpose of increasing a recording density. The optical recording on thin films having the above-mentioned composition do not change their geometrical shapes but change their physical nature, i.e. reflectivity and transmittance, at localized regions therein, and therefore it is also possible to erase the information data previously stored at the localized regions in a suitable manner.
On the other hand, it is also known that optical recording films of Se-Te are similarly applicable for optical recording and reproducing apparatuses capable of erasing stored information data by light irradiation thereon. It has been proposed in the Japanese published unexamined patent gazette No. Sho 53-148244 to provide optical recording and reproducing apparatus by employing optical recording films of Se-Te and using three laser light wavelengths. In the invention of this gazette, the employed wavelength of the reproducing beam light is selected to be longer than that of the beam light used for the erasing step. The disclosed optical recording and reproducing disc apparatus is also capable of erasing the previously optically stored information data in the recording disc films. The disclosed recording and reproducing disc apparatus utilizes a photo-irradiation effect at the erasing step in order not to heat the optical recording disc films, although the optical recording disc films are heated at the recording step upon light irradiation. However, it has been reported that such a photo-irradiation effect can be expected only for a wavelength range around 500 nm. Accordingly, it is necessary to employ bulky gas laser apparatuses such as Ar gas ion laser apparatus, since it is so far impossible to lase existing semiconductor laser devices at a wavelength range around 500 nm. In addition, the above-mentioned optical recording and reproducing disc apparatus has a shortcoming that the recorded information data stored in the optical recording disc films would be easily erased under sunlight, as far as the erasing process is effected by the above-mentioned photo-irradiation effect available around 500 nm. In order to overcome such a shortcoming, it is necessary that both recording and erasing processes are effected under a heating effect upon light irradiation upon an optical recording disc film. Then, the optical recording disc film, when suitably selected, is capable of stably storing the stored information data even under sun light, since heating energy produced by sun light irradiation would thermally diffused inside the optical disc substrate coated with the optical recording disc film.
For the several reasons described above, it is highly demanded to provide a compact optical recording and reproducing apparatus, which is capable of recording information data in optical recording disc films with a high density, and of reproducing the optically recorded data immediately after the recording or after recording, and further capable of erasing the stored information data. This is possible by employing semiconductor laser devices as recording, reproducing and erasing light beam sources, and by employing recording disc materials capable of both stable recording and erasing information data by heating caused by light irradiation upon the recording disc materials.